Typical Phillips-type activated catalysts for use in the polymerisation of olefins comprise chromium oxide carried on surfaces within the pore structure of porous inorganic support particles. For many uses, aluminium oxide, titanium oxide, or other oxides may also be present, either as part of the inorganic support itself (for instance where the support is a silica-alumina co-gel support), or also carried, along with the chromium oxide, within the pore structure of the porous inorganic support.
Typically, catalyst compounds are impregnated, in liquid form, into the pore structure of porous support particles. For instance, the catalyst compound may be impregnated into the pore structure as an organometallic compound, for instance dispersed or dissolved in a solvent. Solvent removal may be carried out by evaporation prior to activation of the catalyst compound. The impregnated support particles (i.e. inorganic porous support particles impregnated with catalysts compounds) are referred to as catalyst precursor particles and subsequently may require activation in order to make them useful as catalyst particles, for instance for olefin polymerisation.
Generally, the carrier particles, after impregnation with the catalyst compound, are supplied and transported in unactivated form (i.e. as catalyst precursor particles), and may require activation prior to being useful as catalyst particles for olefin polymerization. Activation is carried out by heating the catalyst precursor particles at a high temperature, such as from 200° C. to 1200° C., preferably from 400° C. to 1000° C. for a time, from a few seconds, but typically up to several hours, for instance from 30 minutes to 15 hours, in a non-reducing atmosphere such as nitrogen, inert gas or carbon dioxide, or preferably in an oxidising atmosphere such as air or oxygen, so that the catalyst is converted into an active state. For instance chromium is suitably converted to a chromium VI state by oxidation. Once activated, the catalyst may be used at once, or stored in a dry, inert atmosphere until it is used.
It is desirable for the support particles to have a pore structure with a high pore volume. For instance, a pore volume of 1.5 cm3/g or more is desirable. It is also desirable for the carrier particles to exhibit a high surface area, for instance 600 m2/g or more.
Mean pore diameter for the pore structure of a support particle is proportional to pore volume and inversely proportional to pore surface area. Hence, it is possible in principle for two carrier particles to have the same mean pore diameter, but with the first support particle having low pore volume and low surface area, and the second support particle having high pore volume and high surface area. It is desirable that the pore diameter for the pores making up the pore structure should be large in order to obtain an active catalyst (there is a minimum pore diameter requirement for the catalyst to be active according to prior art literature). Polymerisation processes may be targeted to produce polymer of a certain selected molecular weight. This is usually assessed by the Melt Index (MI) of the polymer, with the MI having an inverse relationship with molecular weight (high MI indicating low molecular weight, and vice versa). For a fixed MI target and a fixed catalyst pore diameter, higher pore volume is associated with higher surface area, higher activity and also with improvements in Environmental Stress Crack Resistance (ESCR) for instance as measured by NCTL (Notched Constant Tensile Loading) behaviour for the polymer generated by polymerisation using the support particles. Catalyst activity is a parameter indicating the weight of polymer generated per weight of catalyst used per hour, so a high value is desirable for reasons of efficiency.
If two catalyst particles having the same pore volumes, but differing pore diameters, and hence differing surface areas, are compared: the catalyst particle with the higher surface area will have a lower pore diameter, and for the same activation and polymerisation reactor conditions, will yield a lower MI polymer than the catalyst particle with a lower surface area. The pore volume and surface area for a catalyst are closely related to the corresponding values for the support particle used to make it (for instance, depending upon treatment conditions, the surface area may be up to say 150 m2/g lower than that of the corresponding support particle and pore volume may be reduced by up to say 0.8 cm3/g compared to the pore volume for the support particle). In other words, the provision of a support particle with a fixed pore volume and a higher surface area for its pore structure allows for the polymerisation of olefins to yield low MI polymers (high molecular weight polymers) at high activity and with good NCTL/ESCR behaviour for the resulting polymer.
High load melt index (HLMI) and melt index (MI) are polymer characteristics determined in accordance with ASTM D-1238 using loads of 21.6 kg and 2.16 kg respectively at 190° C.
In conventional manufacturing processes for preparation of carrier particles based on silica, a silica hydrogel is prepared and then water removed from the hydrogel to yield a dried gel or xerogel having a pore structure left by the removed water. For instance, a reaction between an alkali metal silicate, such as sodium silicate, and an acid may be carried out to form a hydrosol, followed by gelation of the hydrosol to yield a hydrogel. Typically, the hydrogels for use as catalyst support particles are aged at a temperature of 40° C. or more for several hours or more, and at a pH typically greater than 7. This ageing step has been considered necessary in the prior art in order to sufficiently strengthen the resultant silica hydrogel so that subsequent removal of the water from the hydrogel, to provide the pore structure, does not lead to the silica skeleton of the particles collapsing during water removal and yielding a low pore volume. However, this ageing process, when used to obtain a final pore volume of 1.5 cm3/g or more, leads to an accompanying reduction in surface area to values less than 600 m2/g.
The theoretically attainable pore volume will be determined by the solids content of the final hydrogel after washing, with water filling the pore structure, and hence the solids content of the hydrosol from which the hydrogel is formed.
In this specification, the term “solids content” as applied to hydrogel refers to the weight percentage of insoluble oxide solids in the washed hydrogel, i.e. with soluble salts substantially removed from the pore structure. For a silica hydrogel with a skeleton consisting of silica only, this will correspond to the silica content, but when other insoluble oxides such as titania or alumina are also present within the skeletal molecular structure of the silica, the total amount of insoluble oxide should be taken into account.
The theoretical maximum pore volume attainable will be determined by the volume of insoluble solids in the sol/gel. Were the solids to form a completely rigid skeletal network which did not collapse when liquid was removed from the gel, then all of the volume vacated by liquid would remain as pore volume for the resulting catalyst support particles. Typically, the theoretical maximum pore volume (TPV in cm3/g) is estimated by (100−% SiO2)/%/SiO2, for a silica gel, where % SiO2 is the percentage by weight of SiO2 in the washed silica hydrogel (i.e. with substantially no soluble salts in the pore structure of the hydrogel).
Without wishing to be bound by any theory, it is thought that the removal of water from the pore structure of a silica gel leads to forces, arising from the surface tension of the water, collapsing the pore structure at least in part as the water is removed. This effect may be reduced by solvent exchange methods, where the water within the filled hydrogel is first partially or entirely replaced by a solvent having a lower surface tension than water (such as an aliphatic alcohol, e.g. methanol or ethanol, or for instance propan-2-ol, trifluoroacetic acid or acetone). Solvent exchange methods are well known in the field and include azeotropic distillation and multiple solvent exchange processes where a first solvent exchange is carried out with a water-miscible first solvent and a subsequent exchange with a water-immiscible second solvent that is miscible with the first solvent. Although such solvent exchange processes may reduce loss in pore volume resulting from drying the hydrogel (for instance by first exchanging solvent for water and then removal of the exchanged solvent) it may not eliminate pore volume loss completely, and so strengthening of the porous silica structure is still important if pore volume is to be maintained when water is removed from the pore structure of the hydrogel. Solvent exchange processes, when used to obtain a final pore volume in excess of 1.5 cm3/g, still require ageing of the hydrogel prior to water removal which typically leads to an accompanying reduction in surface area to values less than 600 m2/g.
It is desirable for catalyst support particles to be sufficiently friable that the particles break down during olefin polymerisation processes in order to avoid a gritty texture in the resulting polyolefin. The fragments of catalyst particles are typically allowed to remain in the resulting polyolefins and so it is important that they break down into tiny particles which are dispersed throughout the polyolefin. However, catalyst support particles which are too friable may be unable to withstand the processing steps used to deposit catalyst metal into the pore structure and to form activated catalyst particle.
Ageing at pH greater than 7 has typically been the conventional method used to achieve an adequately large pore diameter for xerogel catalyst support particles. Ageing is thought to result from a strengthening of the silica matrix or skeleton to resist collapse, and it is thought that the strengthening arises from Ostwald ripening of the silica particles, so conventionally, conditions are selected (i.e. pH greater than 7) so that the silica is relatively soluble in order that such ripening may occur within reasonable times. In practice, it has been found that the increased pore volume arising from such strengthening has to be traded off against an accompanying reduction in surface area. This typically results in surface area values less than 600 m2/g for the catalyst support particles having pore volumes of 1.5 cm3/g or more and prepared by solvent exchange of hydrogel aged at pH 7 or more.
Attempts to overcome these problems in the prior art have included the use of a supercritical fluid, such as organic solvent or carbon dioxide, for replacement of water in the hydrogel. The principle underlying the use of supercritical solvent is that the surface tension vanishes for a solvent in a supercritical state and so strengthening by ageing at pH greater than 7 may be obviated. Silica gels with a surface area of 1195 m2/g and with a pore volume of 3.53 cm3/g were achieved by use of supercritical fluid as an exchange medium. See Ranliao Huaxue Xuebao 1996-24(6) 517 to 521. See also Yang Ru et al, Microporous and Mesoporous Materials, 129(2010) 1 to 10. However, the use of supercritical fluids typically requires high pressure containment vessels and is a highly complex process for safe operation on an industrial scale. Furthermore, the resulting xerogels may be weaker than conventionally strengthened xerogels.
Alternatively, the surface of the silica may be treated with a silylating agent prior to formation of a xerogel. This is described for instance in U.S. Pat. No. 7,470,725 and in Microporous and Mesoporous Materials (2008) 112 (1-3), 504 to 509 or Applied Surface Science (2007), 254(2) 574 to 579. The use of a silylating agent leads to the majority of the silica carrier's surface hydroxyl groups being eliminated due to reaction with the silylating agent. Hence the dried gels typically have a high carbon content. For example, the silica carrier prepared according to U.S. Pat. No. 7,470,725 may contain as much as 12% by weight of carbon in the dried state. For many catalytic applications, the surface hydroxyl groups of the silica may have an important role to play in anchoring catalytically active species, and so their chemical inactivation by silylating agents is undesirable.
Another route which has proved effective, for giving support particles with high surface areas and high pore volumes, is freeze-drying of hydrogels. This involves freezing at a temperature sufficiently low for the water in the hydrogel pores to be maintained in a solid state, followed by vacuum sublimation of water from the hydrogel. This is disclosed, for instance, in U.S. Pat. No. 3,652,214 in which the resulting silica carrier has a surface area of 757 m2/g and a pore volume of 2.77 cm3/g. The freeze drying process is a slow and energy intensive process compared to solvent-exchange based formation of xerogel catalyst support particles.
Another prior art route requires the presence of a small quantity of polymerisation modifier to be present in the solution/sol from which the hydrogel is formed. This method, disclosed in U.S. Pat. No. 5,229,096 provided a silica carrier, after washing, with a surface area of 995 m2/g and a pore volume of 2.32 cm3/g. The polymerisation modifier may remain as an impurity in the carrier and could have a negative effect on catalyst performance if not entirely removed from the catalyst support particles.
Hence, there is a need for a process for preparation of porous xerogel catalyst support particles which have both a high surface area and a high pore volume for the pore structure. The method should also be suitable for being carried out using current processing equipment and not require the introduction of potential catalyst contaminants into the xerogel during its preparation. There is also a need for a process for preparation of such porous xerogel catalyst support particles so that the particles have suitable strength to provide resistance to transport and handling but sufficient friability to break down during use in polymerisation reactions.